Modify makeKey function to use Array.map instead of for loop.
Add createImage.
Add test for randPictureUrl.
Modify isImageExistTest not use Image and workArea but use createImage.
Add cmdShowRandomImage.
Rename function cmdShowRandomImage to cmdSetImageRandomUrl.
Add cmdShowNewImage.
Add cmdToggleStartButton.
Complete cmdToggleStartButton test.
Refactoring: modify button value to constant, extract common code as a function.
Refactoring: extract common code as a inner function.
Add imageOnError.
Refactoring: extract common code as some functions.